Human adenoviruses (HAdV:s), which belong to the mammalian adenovirus genus (Mastadenovirus), are commonly encountered infectious agents. In human, adenoviruses are associated with various clinical symptoms including ocular diseases, such as conjunctivitis and epidemic keratoconjunctivitis (EKC).
To date, there are unfortunately no specific antiviral drugs available for the treatment of adenovirus infections. Adenoviruses are obligate intracellular parasites thus indicating that they are fully dependent on the cell's replication machinery. The selective inhibition of adenoviruses replication by antiviral compounds is therefore very difficult to achieve as some of the essential functions of the host cells may also be altered. However, one approach in today's antiviral drug research consists in blocking the cellular receptors of the viruses so that their attachment to and penetration of the cells are precluded. No drug based on such blocking for use in the treatment of epidemic keratoconjunctivitis (EKC) has been registered.
Adenoviruses are ubiquitous in nature and, therefore, new serotypes are still being discovered. Thus, about 60 years after the isolation of the first HAdV:s, over 50 new serotypes that are grouped into seven species (A-G) have been identified.
EKC is a severe and highly contagious ocular infection that is contracted by millions of individuals each year. Among the adenovirus serotypes responsible for EKC, HAdV-8, HAdV-19 and HAdV-37 remain the principal causative agents of the infection, but recently also HAdV-53, HAdV-54 and HAdV-56 have emerged as novel EKC-causing types. Associated symptoms are keratitis, conjunctivitis, edema, pain, lacrimation, formation of pseudomembranes and decreased vision. Because these viruses are spread by contact (e.g. hand to eye contact), EKC is frequent in densely populated areas and in medical wards with insufficient hygiene precautions. The infection commonly last for up to two weeks; however, some patients continue to suffer from sight impairment for several months, years or even permanently.
The viral life cycle is initiated by the binding of adenoviruses, via their homotrimeric fiber knobs, to sialic acid-containing glycans that are situated on epithelial cells in the cornea and/or conjunctiva. The fiber knobs, located at the most distal part of each of the 12 fibers that are protruding from the adenovirus virion, hold the carbohydrate recognition domains. Lately, glycoproteins with glycans corresponding to the glycans in the GD1a gangliosides were evidenced as functional receptors for the infection of ocular cells by EKC-causing adenoviruses. The crystal structure of the HAdV-37-GD1a complex showed that the terminal sialic acid residues located on each of the two branches of the GD1a glycan were accommodated into two out of three carbohydrate recognition sites on top of the HAdV-37 fiber knob.
Thus, inhibition of adenoviruses with natural or synthetic sialic acid derivatives may prevent the virion to attach to, penetrate into and infect new cells (cf. WO 01/037846 among others). As a result, the infection would become limited. Importantly and especially in the case of EKC, the poor pharmacologic properties of carbohydrate-based drugs that include rapid serum clearance and poor cellular uptake can be bypassed by the use of a topical mode of administration (e.g. cream, ointment, eye drops).
WO 01/037846 discloses that adenoviral infections and in particular ocular adenoviral infections, e.g. kerato-conjunctivitis, may be treated or alleviated by the administration of a substance, interfering with the interaction between the virus and the sialic acid receptor, such as sialic acid, in a therapeutically effective amount. Unfortunately the weak interactions between carbohydrates and proteins limit the use of carbohydrates as drugs.
Attempts to overcome said limitations have been made by using a glycoconjugate with several sialic acid derivatives linked to human serum albumin (SA-HSA). However, such polyvalent glycoconjugates are for several reasons not suitable as pharmaceuticals. The exact structure and composition of SA-HSA will vary between different molecules. Accordingly, SA-HSA represents a type of structure, which is hard to structurally define. Furthermore, the composition of the SA-HSA derivatives will vary between different batches even if produced in the same manner. From a safety and a regulatory perspective this is a significant drawback. In addition the use of a protein, i.e. HSA, which is derived from human plasma, is a major disadvantage. The origin of HSA makes it hard to produce larger amounts of a pharmaceutical based on HSA. Furthermore, contamination by infectious agents, such as viruses or prions, may not be excluded in HSA derived from human plasma. Accordingly, a product based on HSA, is not suitable as a pharmaceutical product, and a polyvalent alternative would be highly desirable.
In WO 2011/003876 novel amphiphilic sialic acid derivatives forming multivalent aggregates in aqueous solutions are disclosed. The aggregates are disclosed to overcome the drawbacks associated with SA-HSA, thereby being by useful in treating EKC. Further aspects of such derivatives have been disclosed by Aplander et al in J. Med. Chem. 2011, 54, 6670.
Further, covalently bound multivalent efficient sialic acid-based inhibitors of HAdV-37 infection of human corneal epithelial (HCE) cells have been reported in the art (cf. Spjut et al. Angew. Chem. Int. Ed. 2011, 50, 6519; Johansson et al. Chembiochem 2005, 6, 358; and Johansson et al Antivir. Res. 2007, 73, 92). In order to circumvent the relatively low efficacy of monovalent sialic acid derivatives, the authors took advantage of the trimeric binding site at the HAdV-37 fiber knob. The use of tri- and tetravalent sialic acid derivatives that can simultaneously bind to more than one carbohydrate recognition domain per knob, as disclosed by Spjut et al, considerably improved the inhibitory potency in comparison to monovalent sialic acid compounds.
For instance, a compound denoted ME0322, being a synthetic trivalent sialic acid derivative wherein squaric acid is used to link sialic acid to a central core unit, was reported with four orders of magnitude more potent than the natural sialic acid monosaccharide. Further, ME0322 was as effective as 17-valent sialic acid-HSA conjugate in preventing binding of the virus to the fiber knob. Interestingly, ME0322 was found to be far more potent than the HSA-conjugate in inhibiting infection of human corneal epithelial (HCE) cells by HAdV-37.
While said compound indeed shows interesting properties, it would still be useful to provide a multivalent sialic acid derivative having even further improved potency in terms of preventing the attachment of HAdV-37 virions to human corneal epithelial (HCE) cells.
Thus, there is a still a need in the art for a low molecular, multivalent sialic acid derivative displaying high efficacy in preventing the attachment of HAdV-37 virions to human corneal epithelial (HCE) cells. Such a derivative would be useful in treating and preventing EKC.